In applying an electrical signal to an instrument, it is often advantageous to insert a switch between the signal and the instrument, so that the two can be selectvely coupled and decoupled, for example, during the transient periods of the signal, such as its initial slewing and settling periods.
However, most electrical switches have an inherent resistive effect in their "on" state (hereinafter called the ON-resistance, R.sub.on). The ON-resistance of a switch moreover varies in accordance with the voltage level of the applied signal, which, in most applications, may further interfere with the proper operation of an instrument. For example, in "A New Single-Chip C.sup.2 MOS A/D Convertor For Microprocessor Systems--Penta-Phase Integrating C.sup.2 MOS A/D Convertor", IEEE Journal of Solid State Circuits, Vol. SC-13, No. 6, Dec. 1978, page 781, there is disclosed that the ON-resistance of a switch would reduce the conversion accuracy of an A/D convertor.
To further illustrate the undesirability of the ON-resistance of a switch, consider the conventional integrator circuit 1 of FIG. 1. In the circuit, an input signal is coupled to a standard integrator 12 by way of a switch 11 so the transients of the input signal can be avoided.
In the ideal case, the output, V.sub.out, of an integrator circuit should preferably be equal to: EQU V.sub.out =-(1/R.sub.int C.sub.int).intg.V.sub.in dt
where
R.sub.int =resistance of integrating resister 13, PA1 C.sub.int =capacitance of integrating capacitor 12.
The time constant, t.sub.c, of the integrator 12 should equal to R.sub.int C.sub.int. Such an ideal case is usually not attainable in the circuit 10 of FIG. 1. Instead, because of the ON-resistance of the switch 11, the time constant of the circuit 10 becomes t'.sub.c [R.sub.int +R.sub.on ]C.sub.int. The operation of the integrator 12 may degrade further if the ON-resistance of the switch R.sub.on changes further changes in accordance with the level of the input voltages, V.sub.in.
A conventional method of reducing the resistive effect of switch 11 is to use a resistor R.sub.int having resistance much higher than the ON-resistance R.sub.on as the integrating resistor. In this way R.sub.on becomes negligible (i.e. R.sub.int &gt;&gt;R.sub.on, and R.sub.int +R.sub.on .apprxeq.R.sub.int), so that t'.sub.c =(R.sub.int +R.sub.on)C.sub.int .apprxeq.R.sub.int C.sub.int.
However, making the resistor R.sub.int too large may pose another problem. In applications such as analog-to-digital (A/D) converters, the R.sub.int C.sub.int time constant must be kept small to provide fast integration. On the other hand, C.sub.int must be kept relatively large so that the A/D converter can operate accurately. As a result, R.sub.int should preferably have a small value. Hence, the use of a large R.sub.int is not usually desirable, and there exits a need for an electrical switch that has little ON-resistance.